1. Field of the Invention
The present invention relates to a rapid-action frequency tuning device for a microwave tube and to a microwave tube equipped with such a device.
2. Discussion of the Background
This device applies especially to tubes of the klystron family and more particularly to low-power klystrons. These tubes are used in particular as the power stage of a ground station amplifier designed to transmit television or telecommunication signals to a satellite.
Conventionally, a klystron comprises an electron gun which produces a long thin beam of electrons through a body formed by a succession of resonant cavities. Four to six cavities are generally used. A microwave signal to be amplified is injected into the first cavity. The microwave signal interacts with the electron beam and is recovered, amplified in the final cavity, while the electrons are collected in a collector located at the output of this final cavity.
Klystrons, unlike travelling-wave tubes which are also used in ground transmitting stations, do not have a very large instantaneous bandwidth. The bandwidth is of the order of 10 to 100 MHz at most. However, it is possible to make klystrons operate over a wide frequency range and to shift their instantaneous bandwidth, which is much too small, within this frequency range by tuning the central frequency of the tube, that is to say by mechanically adjusting the resonant frequency of one or more cavities of the tube.
A frequency tuning device of known type comprises a tuning element which can be moved inside each cavity that has to be tuned. This tuning element varies either the inductance or the equivalent capacitance of the cavity. This element may be a plunger which varies the internal volume of the cavity or a plate which can be moved in relation to the electron beam. These frequency tuning devices, by means of a manual or motorized selection mechanism, allow a preset configuration of the position of the various tuning elements to be found again, this configuration being called a channel.
In large klystrons, each tuning element is fastened to a threaded rod which extends outside the cavity and which can be screwed on or unscrewed manually. In order to make it easier to set a channel, each of the tuning elements is associated with a counter which counts the number of revolutions of the threaded rod. The information from the counters allows a particular position to be easily found again. The tuning elements are therefore each moved in turn.
Setting all the cavities of a klystron takes approximately 20 seconds and this time is much too long for switching from one channel to another. What is more, the inevitable friction and mechanical hysteresis of the moving parts makes this device somewhat imprecise.
A little time has been saved by using, for each of the channels, a mechanical means of memorizing the position of the tuning elements with the aid of stops. This tuning device comprises several sets of stops on a movable carriage. Each of the sets has one stop per tuning element. The tuning elements bear on the stops of one of the sets. To change channel, all that is required is to retract the tuning elements from the stops with the aid of a suitable device, to change the set of stops by moving the carriage and to reposition the tuning elements so as to bear on the stops of the new set. The tuning elements are therefore moved simultaneously. Elastic elements compensate for the friction and the mechanical hysteresis.
The stop-holder carriage and the device for retracting the tuning elements may be actuated with the aid of motors. This replaces direct human intervention and saves time. However, it is not possible with such a device to change channel in less than four seconds and this time is still regarded as too long. Another drawback of this approach is that this device is mechanically much more complicated and more fragile.
The travel imposed on the tuning elements during their retraction with respect to the stops causes a not insignificant amount of mechanical fatigue of the moving parts. In the motorized version, if an incident occurs, such as the stop-holder carriage being in an incorrect position or a tuning element being blocked, the action of the motors may damage one or more stops, one or more tuning elements or more seriously, one or more resonant cavities.